OPTIMIZATION MODELING OF THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER IN ARKANSAS

摘要

Conjunctive-use optimization modeling was done to assist water managers and planners by estimating themaximum amount of ground water that hypothetically could be withdrawn from wells within the Mississippi River Valleyalluvial aquifer without violating hydraulic-head or stream-discharge constraints. In 1997, nearly 6 million acre-feet of waterwere pumped from the aquifer by more than 45,000 wells, primarily for irrigation and for fish farming. Several large conesof depression over 100 feet deep have formed in the potentiometric surface, resulting in lower well yields and degraded waterquality in some areas. MODFLOW-based ground-water flow models were developed for the north and south portions of theaquifer, primarily in eastern Arkansas. MODMAN optimization models based on these flow models showed that continuedpumping at 1997 rates are unsustainable without violating head constraints imposed as a part of Arkansas’ Critical Ground-Water Area criteria. One of these criterion states that if ground-water levels drop below half the original saturated thicknessof the formation, then a “critical ground-water area” may be designated. In addition, streamflow constraints were establishedbased on minimum flow requirements for maintaining water quality and fish habitat. Preliminary results show thatcontinuous pumping at 1997 rates indefinitely resulted in water levels dropping below the half-thickness constraint, makingthese rates unsustainable. Optimized sustainable pumping was obtained such that water levels were maintained at or abovethe half-thickness constraint or a total saturated thickness of 30 feet, and streamflow was maintained at or above minimumlevels. Optimized sustainable yields from streams were nearly 2 orders of magnitude greater than for ground water.